Material presses of this general type are known. One such known press comprises a drivable central drum, the temperature of which is controllable. An endless steel belt passes over a tensioning drum and at least two guide drums as well as the central drum. The material to be processed is guided between the tensioned steel belt and the external surface of the central drum whereby the material is pressed in a temperature-controlled manner. Additional pressing means are disposed adjacent a portion of the periphery of the central drum on the side of the steel belt remote from the material being treated.
Such a press permits the production of thin products, such as webs of paper, textile, fibreboard and chipboard to be achieved. Moreover, the use of such a press as a cross-linking device for webs of rubber or thermoplastic materials has also been known for a long time. By way of contrast with a calendering system, such presses are generally used when the application of a temporary line pressure as the web of material passes between two rollers is not sufficient to sub the material to the desired cross-linking conditions.
However, when such presses are used for producing chipboards of relatively large thickness and/or high material density, the maximum pressure which can be applied and which is dependent upon the tensile strength of the steel belt, has proved to be too low. Proposals hitherto for increasing the pressure which can be applied by the steel belt are not, however, suitable for continuously producing chipboards or fibreboards having a high density. In consequence, it has long been desired to be able to manufacture boards having a better homogeneity and a better density profile over the board thickness, so that mechanically more stable products can be produced. However, this requires a considerably higher pressing force to be applied to the web of material than has hitherto proved possible using known arrangements.
In German Auslegeschrift No. DE-AS 17 04 767, an arrangement which attempts to overcome this problem is disclosed. In such arrangement, the steel belt acting on the material is pressed thereagainst by a pressure plate. The plate has a substantially semicircular configuration and acts on the steel belt from externally. Such plate is connected to an arcuate carrier through the intermediary of a plurality of hydraulic pressure piston and cylinder arrangements. The carrier is supported on the bearing shafts for the central drum by means of lateral plates, so as to be rotatably mounted thereon. The pressure plate is pressed against the steel belt by means of the hydraulic piston and cylinder arrangements and, in consequence, the pressure acting on the strip of material can be increased.
This arrangement is, however also disadvantageous, in that the entire pressing means must move with a steel belt during rotation of the central drum. It must then be returned to its initial position at the end of a pressing cycle when both the central drum and the steel belt are stationary. The pressure plate is then replaced in position on the steel belt and pressed thereagainst so that the central drum, the steel belt and the pressing means can process an additional portion of the strip of material. Such an arrangement can therefore only be used to produce endless webs of material in an intermittent and, hence, time-consuming and cost-intensive operation. In addition, the provision of a solid pressure plate only permits the processing of materials which do not suffer large reduction in their web thickness due to their pressing between the surface of the central drum and the steel belt. The reduction in thickness, which usually occurs during the production of chipboards and fibreboards, causes, even with thick webs of material, a visible reduction in the required radius of curvature of the pressure plate when a surface-to-surface pressing operation is also required. It is true that adaptation to the varying radius of curvature can be substantially achieved by the hydraulic piston and cylinder arrangements but this impairs the material strength of the pressure plate which is permanently loaded in a non-uniform manner. In addition, the possibility of the pressure plate becoming locally deformed cannot be excluded, so there can be no guarantee that the pressing force will act uniformly over the entire contact surface. It is, of course, the uniformity of application of such force which determines the quality of the end product. Finally, this arrangement requires a considerable amount of material, particularly if the pressure plate is to be pressed against the steel belt over the entire roller width by means of hydraulic piston and cylinder arrangements which are disposed in a grid-like array as would be the case if it is desired to eliminate the use of a steel belt entirely so as to preclude deformation of the end product.
Similar disadvantages are also to be found in the arrangement disclosed in German Auslegeschrift No. DE-AS 23 11 909. Such prior document discloses the application of the additional pressing force to the steel belt by means of an endless roller belt which is acted upon by a tensioning device comprising two half-shells. In such arrangement, the roller belt is guided by means of guide rollers and tensioning rollers onto the outer surface of the steel belt and rotates around the central drum with the steel belt.
Two half-shells, which are interconnected by means of an adjusting lever and tensioning device, cover the roller belt and exert a pressure thereon. This, obviously, increases the pressing force which acts on the web of material through the intermediary of the steel belt.
Since the maximum transferable pressing force depends on the tensile strength of the belt and of the half-shells, the required high force cannot be exerted upon the strip of material utilising such an arrangement. In addition, the strip of material experiences a disadvantageous, sudden reduction in the pressure applied thereto approximately after halfway along its circumferential path around the central drum. This pressure reduction occurs in the region of the junction between the two half-shells, because the half-shells do not abut one another in a flush manner in such region.
In consequence, it has not hitherto proved possible, utilising known additional pressing means, to produce webs of material having different final thicknesses. This is because the radius of curvature of the pressure plate is selected in dependence on the value which results from the sum of the central drum radius, the end thickness of the strip of material, the thickness of the endless steel belt and, possibly, the height of the roller chain.